Wednesday, 28 May 2008

Shell suit

One of my solar-powered garden lights has become home to a colony of snails. Typically about 20 congregate there every day, mainly on the north face of the light (see Photo), but none on the solar panel. Are they adapting to 21st-century technology and using solar power to extend their periods of access to warmth and light?Roger Bloor, Newcastle-under-Lyme, Staffordshire, UK

Well, nobody has replied, so I'll share what little I know.I found some information on snail behaviour years ago, and I think it said that they're most active at night, retreating into their shells during the day.As morning approaches and the amount of light is starting to increase, they start looking for a place to spend the day. Ideally, they want a small, moist cave, although the underside of a large leaf or overhang will do nicely. So they tend to move towards darkness (cave and shadow) and upward (clinging to the underside of something makes it less likely for them to be accidentally found and eaten by critters).I note that the light's stem and top are black. This makes sense if my recollection is true, as they head for the dark stem, start climbing up, and when they reach the clear part, they can still 'see' the 'darkness' of the cover, so they keep going until they reach the underside of the top cover.Some keep going, others stop under the overhang. But overall, they move to be away from the sun. They seem to be sensitive enough to mostly go to the north side.For fun you could mark some of the shells with white-out or nail polish (snail polish?) and see if the same snails come back to the same lamp.

If you are going to mark the snails with tipex (aka white-out) or nail varnish then please think about the fact that you will be advertising the snails to predators. It would be better to mark them with a non-toxic uv pen, and then use uv light to look at them; or to use a nail varnish that is similar to the shell colour so that it cant be easily spotted.

The following answer has been selected and edited by New Scientist staff

The snail pictured is the garden snail, Helix aspersa or Cornu aspersum, which was accidentally introduced into England, probably in Roman times, by trade with mainland Europe. Among other habitats, it is associated with disturbed areas such as gardens and waste ground.

Avoiding loss of water is a major priority in the life of a snail, and is perhaps the single most important factor influencing their day-to-day activity and behaviour. As a result snails are essentially reclusive animals, spending much of their lives hidden away in sheltering microhabitats. Gardeners will be well aware that they can be found in abundance under bricks or stones.

Garden snails commonly come together in places that offer some type of protection, in particular to undergo aestivation and hibernation. Aestivation is a period of inactivity in dry weather during which snails avoid losing moisture by withdrawing into their shell and adhering to a substrate via a mucous membrane. Hibernation occurs in winter and involves the shell being sealed with a calcareous plate, or epiphragm, to avoid water loss, while the snail's pulse rate is reduced.

In the situation your questioner describes, by choosing the north side of the solar-powered light, the snails both avoid exposure to the sun and are able to shelter in the artificial "crevice" afforded by the lip of the solar panel. They would naturally avoid the top surface of the panel, which is much more exposed and, of course, faces the heat of the sun.

During warmer months the snails will often be active away from the lamp, returning to their "roost" by following the chemical signals in their slime trails. At night, it is possible that the light actually provides them with some warmth. However, snails would normally avoid the desiccating heat and light given out by a more powerful lamp.

The following answer has been selected and edited by New Scientist staff

Snails' preference for smooth, sheltered surfaces forms the basis for a good snail trap requiring no poison. Where snails are most plentiful, stand plastic flowerpots upside down with their rims raised about 25 millimetres off the ground on the side facing away from the sun. After a spell of rain following dry weather, you can collect these garden guests by the dozen for donation to grateful hedgehogs and neighbours.

Since my comment was published in New Scientist I have had some "snail mail" (!) feedback regarding a couple of errors that I made (not, however, directly related to the question). Firstly, the epiphragm of C. aspersum is made of proteinaceous chitin and is not calcareous, as it is in its larger relative, Helix pomatia. Secondly, although it was thought that C. aspersum was probably introduced during the Romano-British period, subsequently semi-fossil shells have been discovered in Crag deposits, indicating that it's introduction may have been much earlier, possibly during the bronze age.

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